(1) Field of the Invention
The invention relates to image capture. More specifically, the invention relates to association of a plurality of storage elements with a photosensitive element.
(2) Background
Image sensors have become increasingly common in recent years and digital image capture has made great strides. Typically, digital images are derived from image sensing arrays made up of rows and columns of pixels. Each pixel stores a charge corresponding to the light levels at a point on the surface of the image sensing array at an instant in time or possibly an average over some period of time. FIG. 1 shows a schematic diagram of one example of a pixel. The light sensitive element 10 is disposed so as to collect incident light. The light sensitive element 10 may, for example, be a photodiode. A reset transistor 12 is coupled in series with the photosensitive element 10 between the photosensitive element and the power supply Vcc. The gate of reset transistor 12 is driven by a reset signal which when asserted causes the voltage at a sampling node 20 between the reset transistor 12 and the light sensitive element 10 to be a known voltage, in the figure Vcc. A sampling transistor 14 is coupled between the sampling node 20 and a node 22. Storage element 16 is coupled between capture node 22 and ground. The voltage at collection node 22 drives the gate of readout transistor 18.
A sample signal drives the gate of sampling transistor 14. When the sample signal is asserted, the current corresponding to the light level at the light sensitive element 10 at that time appears at capture node 22 and is captured by storage element 16, which may be a capacitor. When the sample signal is deasserted, transistor 14 is off and current does not flow between sampling node 20 and capture node 22. Thus, the charge on storage element 16 theoretically remains constant until the sample is reasserted causing the voltage at capture node 22 to match the voltage at sampling node 20. When the enable signal that drives the source of readout transistor 18 causes the readout transistor 18 to turn on, by exploiting knowledge of the operation of the transistor in the linear region, a representation of the light level of the image previously stored by light sensitive element 10 is read out on the bitline.
While image sensing arrays instantiated using this pixel configuration have been reasonably successful, they are not particularly effective in video application or where moving subjects are involved. This is caused in part by the fact that the read out time can be quite significant and, therefore, necessitates a time delay between exposures. As a result of this shortcoming, the options are to limit the motion of the subject of a digital photograph or use post-processing to move the object back to where it should have been had it been possible to obtain exposures closer in time.
Additionally, dynamic range of such pixels may be relatively small. Dynamic range is defined by the amount of noise and the transfer function of the photoelectrons generated by the light. A base noise level fundamental to the device is determined and the signal corresponding to the generated photo electrons is added on top. As noise increases, there is less room to add signal and the signal to noise ratio decreases. The signal to noise ratio is also related to time as longer sampling times result in more noise and more signal, while shorter sampling times result in less noise but also less signal. Thus, noise reduction is particularly important because of its effect on the dynamic range.
It would be desirable to be able to increase the dynamic range and reduce noise effects as well as reduce problems related to digital capture of objects in motion.